1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus and an optical scanning apparatus which is mounted in the electrophotographic image forming apparatus, such as a digital copying machine, a laser beam printer, or a facsimile apparatus, and which is adapted to perform scanning with a laser beam.
2. Description of the Related Art
Conventionally, an electrophotographic image forming apparatus contains an optical scanning apparatus having a polygonal mirror to deflect a light beam emitted from a semiconductor laser constituting a light source such that the light beam scans a photosensitive drum as a photosensitive member. In an electrophotographic image forming apparatus, a light beam corresponding to image information is emitted from an optical scanning apparatus and applied to a photosensitive drum whose surface is charged, and scanning is performed on the photosensitive drum with the light beam, to form an electrostatic latent image on the photosensitive drum. The electrostatic latent image formed on the photosensitive drum is developed with a developer. The developed image is transferred and fixed to a recording medium such as a paper sheet.
In such an optical scanning apparatus, a rotation shaft for rotating the polygonal mirror may be inclined with respect to a desired installation angle for the rotation shaft (Hereinafter, this inclination will be referred to as shaft inclination). The shaft inclination is an inclination caused by limitations in production precision. If the polygonal mirror has shaft inclination, the incident position and incident angle on a scanning lens may uniformly deviate from the design values, which lead to a deterioration in image formation performance, such as disturbance of the light beam spot shape on the photosensitive drum, thus resulting in degradation in image quality.
FIGS. 8A and 8B illustrate an example of a conventional configuration of an optical scanning apparatus. FIG. 8A is a side view of a polygonal mirror and its periphery, and FIG. 8B is a perspective view of the polygonal mirror and its periphery.
In the optical scanning apparatus illustrated in FIG. 8A, a drive motor 803 for driving a polygonal mirror 804 has a bearing 802. The bearing 802 is fit-engaged with a hole provided in a board 801 (hereinafter referred to as the drive board) on which a drive circuit for driving the drive motor 803 is mounted, whereby the drive motor 803 is mounted on the drive board 801. Further, the bearing 802 is fit-engaged with a positioning hole 808 provided in an optical box 807 of the optical scanning apparatus, whereby positioning is effected on the drive motor 803 with respect to the optical box 807. The bearing 802 bears the shaft of the rotor portion of the drive motor 803, and a polygonal mirror 805 is mounted on the rotation shaft 804 of the rotor portion. The polygonal mirror 805 is fixed to the rotor portion from above by a plate spring or the like.
The drive board 801 is fixed to the optical box 807 by passing screws through screw holes provided in bosses 809, 810, 811, and 812 and through optical-box-side fixation holes 813, 814, 815, and 816 and by tightening the screws.
Since the bearing 802 is mounted on the drive board 801 by swaging or the like, there may be variations in the angle of the rotation shaft 804 with respect to the drive board 801 due to the production precision of the members and the precision of the swaging. Further, when the drive board 801 is formed of sheet metal, there is possibility of the drive board 801 being warped. Then, the rotation shaft 804 on which the polygonal mirror 805 is mounted is tilted accordingly (shaft inclination), and the reflection surface of the polygonal mirror 805 is also inclined, and so the reflection angle in the sub-scanning direction of the reflection light may deviate from the ideal position, thus resulting in deterioration in optical characteristics.
Further, it is also difficult for the mounting bearing surface of the optical box 807 to be machined into a flat surface in a strict sense, and this deviation from the ideal flat surface also leads to shaft inclination of the polygonal mirror.
To address these problems, Japanese Patent Application Laid-Open No. 2005-201941 discusses an apparatus capable of correcting shaft inclination. The apparatus discussed in Japanese Patent Application Laid-Open No. 2005-201941 is equipped with a mechanism pressing a drive board against a bearing surface formed on an optical box via a spring to vary a screw tightening amount, thereby reducing shaft inclination.
However, in a case where the drive board is fixed to the optical scanning apparatus via a sprig as in Japanese Patent Application Laid-Open No. 2005-201941, if the reaction force of the spring is weak, the polygonal mirror cannot be firmly fixed in position with respect to the optical scanning apparatus main body due to rotational vibration generated at the time of rotation of the polygonal mirror. When the polygonal mirror is not firmly fixed in position, the polygonal mirror vibrates due to the vibration generated at the time of rotation of the polygonal mirror, and, due to the influence of the vibration, there is a fear of the image forming position of the laser beam on the photosensitive drum deviating from the ideal position.
Thus, if the polygonal mirror is to be fixed to the optical box with a force strong enough to withstand the rotational vibration, there arises of necessity a need to secure a large spring deformation amount. Thus, to generate a large force, the size of the spring must be increased, which is an obstruction to a reduction in size of the optical scanning apparatus.